The feed of the wire electrode to the welding gun is effected, among other ways, by means of a welding current cable constructed as a hollow conductor. Inside the welding gun the cable is connected to a supply tube to the outer end of which is fixed a holder for the guide tube. The welding current is supplied by way of the nozzle to the wire electrode which is guided through a bore in the guide tube. In order to obtain good electrical conductivity the guide tube is produced from a hard copper alloy. The guide tube, which is situated close to the welding point during the welding process, heats up considerably through the heat radiated from the welding point, through the heat transmitted from the wire electrode to the guide tube and through the relatively strong welding current which flows therethrough. In the known guide tubes the temperatures recorded at the tips reach 500.degree. - 600.degree. C. The effect of this marked heating of the guide tube is that its conductivity drops to fractions of the original. The consequence is that the heat losses caused by the current flow in the guide tube become very great. The poor conductivity also entails a deterioration in the transfer of current from the guide tube to the wire electrode, there is a rise in the electrical transition resistance and the wire electrode seizes and sticks to the guide tube. The result is that the wire electrode no longer travels uniformly through the tube but begins to sputter.
In the case of air-cooled welding guns heat is dissipated from the guide tube mainly by way of welding current cable into the air. To keep down the temperature of the guide tube even with fairly high welding capacities, the gun is cooled additionally by water. In spite of this water cooling the guide tubes still reach temperatures of 500.degree. - 600.degree. C. The cause of this, since the heat must first be transmitted predominantly from the guide tube to the guide tube holder, resides in the poor transfer of heat from guide tube to holder. Since the guide tube is a part subject to wear, it must be easily replaceable. The guide tube is usually connected to the holder in a nut and bolt manner. In most cases the guide tube has a screw thread which is screwed into a threaded, cylindrical bore in the holder until the guide tube is pressed firmly by its bearing face against the holder. The transfer of heat from the guide tube to the holder takes place mainly by way of the threaded part. However, this known manner of fixing permits no optimal heat transfer between the tube and the holder. To allow an adequate heat transfer in the case of high welding capacities, the threaded part of the guide tube and the bore in the holder must be made substantially bigger than is required for fixing the guide tube. However, this results in welding guns which are distinctly unwieldy.
The problem underlying the invention is to design the fixture of a guide tube of the type mentioned initially in a guide tube holder in such a way that, for the same space requirement for guide tube and holder, there is a substantial improvement in the heat transfer between the guide tube and the holder by comparison with the known fixtures.